Thermal switch with cycling delay



March 19, 1957 E. s. FRANKLIN 2,736,115

I THERMAL SWITCH WITH CYCLING DELAY Filed April 29, 1954 2 Sheets-Sheet l 24 tum mm IN V EN TOR. fpMo/vp 6 FRAN/fL/N ATTORNEY Marsh 19, 1957 E. a. FRANKLIN 2,786,115

THERMAL swITcH wrm cycmuc DELAY Filed April 29, 1954 a Sheets-Sheet 2 IN VEN TOR. fDMOND 6. FRANKLIN JMR/ZZZ/LOW AT TORNE) United States Patent O THERMAL SWITCH WITH CYCLING DELAY Edmond G. Franklin, Minneapolis, Minn., assignor to General Mills, Inc., a corporation of Delaware Application April 29, 1954, Serial No. 426,538

8 Claims. (Cl. 200-138) This invention relates in general to switches, and pertains more particularly to a switch especially suited for the thermostatic control of appliances and the like.

Difiicu'lties have arisen in the past with too rapid cycling of switch contacts. For instance, where the switch controls an electrical heating means, the thermally responsive member is apt to follow quite closely the temperature of the heated member, and frequent fluctuations in temperature are apt to cause a corresponding frequent opening and closing of the switch contacts. At times the movement of the contacts will be so rapid that they will not have time to close completely or separate fully before their movement is reversed. This, of course, results in arcing and concomitant deterioration of the contacts due to pitting and the like. Also, opening and closing of the switch contacts in rapid succession will tend to overheat and produce wear of such contacts.

One problem that has confronted the industry has been to develop a switch that will produce a positive make and break" of the contacts and which switch will be reliable and capable of withstanding prolonged usage under adverse operating conditions. Such requirements necessitate an exceptionally rugged switch structure, and it is an aim of the instant invention to provide a switch possessing attributes of this character.

More specifically, it is an object of the invention to employ a thermally expandable bridge through which the entire current passing through the switch contacts courses. In this connection, the invention avoids the use of multiple, parallel paths, so that all of the current is compelled to flow through each strut of the bridge whereby the full heating effect from the current is realized. In this way, each strut can be of larger cross-section than otherwise, since as above stated each strut carries the full amount of current, and the ruggedness of the switch is thereby appreciably enhanced. It might be explained that where only one-half of the current courses through each strut, then the cross-section of the struts for a given material must be correspondingly reduced in order to produce sufficient electrical resistance so as to generate adequate heat for causing the necessary longitudinal expansion of the struts. 7

Even though the invention makes provision for increasing the cross-sectional area of the bridge struts, as outlined above, a further precautionary meansure is introduced which provides additional assurance against failure of the bridge while in service. Since the bridge struts serve as slender columns, it can be appreciated that any compression forces act in a direction tending to buckle the struts. Therefore, an airn'of the present invention-is to avoid subjecting the struts to compression, instead placing these members in tension, as the struts are better able to Withstand this latter type of stress.

Other objects will be in part obvious and in part pointed out more in detailhereinafter.

The invention accordingly consists in the features of construction,combination of elements and arrangement ice hereafter set forth and the scope of the application of which will be indicated in the appended claims.

In the drawings:

Figure 1 is a plan view of the thermal switch exemplifying the subject matter of the instant invention;

Figure 2 is a sectional view taken in the direction of line 22 of Figure 1, this view showing one operative position of the switch;

Figure 3 is a simplified view corresponding to Figure 2 but showing a later operative position;

Figure 4 is a view showing a still diiferent operative position;

Figure 5 is a similar view showing still another position that'the switch assumes during the course of a com plete cycle, and

Figure 6 is a detail plan view of one part of the switch.

Referring in detail to the drawings, the member to be heated in a controlled manner has been designated by the numeral 10. Associated with the member 10 is a stacked thermostat embodying the teachings of the present invention, and it will be observed from an inspection of Figure 2 that the thermal switch comprises the tubular post, 12 having a flange 14 integral therewith at its lower end and an upper end that is riveted over at 16. Extending through the bore of the tubular post is a mounting screw 18 which is readily received in a tapped aperture 2% provided in the member 10.

Superjacent the flange 14 is a temperature responsive device, in the form of a bimetallic member 22, carrying an upstanding stud 24 of insulating material at its free end. Its anchored end is equipped with an aperture which encircles the shank of the tubular post 12. Subjacent the riveted end 16 is a radially extending bracket 24a having a reversely bent end 26 for the accommodation of an adjusting screw 28. The adjusting screw is equipped at its lower end with a downwardly projecting insulated stud element 3%, and at its upper end there is provided a manipulating knob 32 by means of which the threaded shaft 28 may be rotated. Further included as part of theadjusting means is a laterally extending lug 34 which engages the upwardly extending extremity 36 formed integral with the bracket 24a to limit thereby the temperature range through which the switch can function.

Having one end thereof circumscribing the tubular post 12 is a resilient switch arm 38 carrying a contact 40 at its free end. Just above the anchored end of the switch arm 38 is a terminal element 42-via which the current may be introduced to the arm 38. Another switch arm indicated in its entirety by the numeral 44 likewise has one end in encircling relationship with the tubular post, there being an aperture 46 by wihch this encirclement is achieved. A horizontally extending element 48, an integral part of the arm 44, contains an aperture 59 of sufiicient size so as to provide adequate clearance with respect to the downwardly projecting stud 30. At the remote end of the horizontal element 4S'from its anchored end is an up wardly inclined strut 52 which connects witha companion downwardly inclined strut 54. The other end of the strut 54 is integral with a horizontal element 56. The horizontal element 56 contains an aperture 58 of sutlicient size to provide adequate clearance with respect to the upstand ing stud 24 mounted on the free end of the bimetallic member 22. The end of the horizontal element 56 whic'l is remote from the end of which is connected to the strut- 54 carries a contact 60 cooperable with the previously mentioned contact 40. Inasmuch as the current passing through the switch arm 44 is intended to course through the struts 52 and 54, a strip of electrical insulation 621's interposed between the horizontal elements 48 and Although the reason for which will be explained inore fully hereinaftenit can be pointed out thistirne'that' greens the struts 52 and 54 have a limited cross section so as to be readily expandable upon the passage of suflicient current through these struts. Of course, it will be appreciated that the exact cross section of the struts 52 and 54 will depend upon the magnitude of current being carried and that if a relatively small amount of current is contemplated, then the cross section must be correspondingly reduced in order that sufficient electrical resistance will be provided by reason of which the struts will heat sufiiciently to produce the necessary longitudinal expansion thereof. Also, it is to be understood that the end of the horizontal element 56 lying subjacent the insulating strip 62 is anchored with respect to the horizontal element 58 so that any expansion of the struts 52 and 54 will result in a vertical movement of the juncture of these struts with each other. Stated somewhat differently, the struts 52, 54, together with horizontal element 56 constitute a triangular bridge structure with the struts forming an obtuse angle respective to each other at their point of juncture.

Further included in the thermal switch construction is a terminal element 64 which is similar to the terminal element 42, this later terminal element serving to supply current to and from the switch arm 44. In order to insulate the arms 38 and 44 with respect to each other a plurality of insulating washers 66, 68 and 70 are sandwiched However an explanation of a complete operational sequence would undoubtedly be helpful facilitating such an understanding. Therefore assuming that the member 10 has cooled to such an extent that the bimetallic member 22 has permitted the contacts 40 and 60 to come into engagement, this being possible by the inherent resiliency of the switch arms 38 and 44 plus the fact that these switch arms are intentionally biased toward each other, current begins to pass through the struts 52 and 54 as well as the resilient element 56. However, owing to the relatively small cross sectional area of the struts they immediately begin to heat and the heat thus generated is instrumental in causing each strut '2 and 54 to expand longitudinally.

As shown in Figure 3 the expansion of the struts 52 and 54 tend to raise appreciably the apex formed at the junc ture of these two struts, since the remote ends of the struts are restrained by the horizontal elements 48 and 58. This results in an increased pressural engagement be tween the contacts 46 and 60 inasmuch as the juncture of the struts 52 and 54 no longer received support from the upstanding strut 24. In. this way, a meeting of the contacts 40 and 60 is responsible for immediately increasing the contact pressure so that an enhanced contact engagement is produced. To afford easy comparison between the lengths of struts 52 and 54 as modified by the transition from Figure 2 to Figure 3, the strut lengths of Figure 2 have been superimposed on Figure 3 and appear in dotted outline, whereas the solid line position represents the changed length of these struts. The relative positions contrasted in Figures 2 and 3 will give some idea as to the pressure increase derived.

After the member 19 has become heated to a desired extent by reason of the current flowing through the contacts 40, 60, and the electrical heating unit (not shown) associated with the member 10, the bimetallic member 22 will have warped upwardly sufiiciently, as shown in Figure 4 so as to cause engagement once again of the upstanding stud 24 carried thereby with the juncture of the struts 52 and 54. Continued warpage will cause the contacts 40 and 60 to separate and this separation immediately produces a cessation of current fiow with a concomitant cool ng and contraction of the struts 52 and 54. The contraction thus produced is responsible for a rather rapid separation of the contacts 40 and 60 for the struts quickly return to their original length as shown in Figure 2.

The increase in separation is clearly pictured in Figure 5 for there the solid line representation of the struts 52 and 54 bears the same length as that shown in Figure 2. Here again, to afford a ready comparison between positions the position of Figure 4 and the new position assumed in Figure 5, the Figure 4 condition has been superimposed upon Figure 5 in dotted outline form. After the contacts have been separated for a sufficient length of time, the member 10 again cools and the bimetallic member 22 accordingly returns to its straight line configura' tion depicted in Figure 2 and thus it will be observed that a complete cycle of operation has been completed.

As many changes could be made in the above construction and many apparently Widely different embodiments of the invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

It is also to be understood that the language used in the following claims is intended to cover all of the generic and specific features of the invention herein described and all statements of the scope of the invention which, as a matter of language, might be said to fall therebetween.

I claim:

1. A thermal switch comprising a pair of cooperable contacts for controlling an electric current, a first temperature sensitive device arranged to effect initial sepa ration of said contacts in response to increasing temperature, means supporting one of said contacts, a second temperature sensitive device associated with the other contact including electrically conductive means which prior to said initial separation is expanded through heating by current controlled by said contacts but which cools and consequently contracts after such separation due to the cessation of said current, thereby causing the gap between said contacts to increase as it cools, said first temperature device being directly engageable with the electrically conductive means of said second device.

2. A thermal switch comprising a pair of cooperable contacts for controlling an electric current, first means including a pair of inclined struts of electrically conductive material and a member anchoring the opposite ends of said struts in a substantially fixed spaced relationship with each other, one of said contacts being electrically connected to one of said struts, second means carrying the other of said contacts, thermally responsive means engageable with said struts adjacent their locus of juncture, and manually adjustable means engageable with said second means, said struts having a cross-section limited with respect to the current controlled by the contacts for causing substantial expansion and contraction of the struts by heating and cooling in response to the changes in current carried by the struts on engagement and disengagement of the contacts.

3. A thermal switch comprising a pair of cooperable contacts for controlling an electric current, first means including a pair of inclined struts of electrically conductive material and a member anchoring the opposite ends of said struts in a substantially fixed, spaced relationship with each other, said member carrying one of said contacts, second means carrying the other of said contacts, manually adjustable means engageable with said second means and thermally responsive means engageable with said struts adjacent their locus of juncture, said struts having a cross section limited with respect to the current controlled by the contacts for causing substantial expansion and contraction of the struts by heating and coolingin response to the changes in current carried by the struts on engagement and disengagement of the contacts.

4. A thermal switch comprising a first switch arm including a'first horizontal element supported at one end, a pair of inclined struts of electrically conductive material connected together at one end, the other end of one of said struts being connected to the other end of said horizontal element and a second horizontal element having one end thereof connected to the other end of the other strut, the other end of said second horizontal element being insulatingly anchored to the first horizontal element adjacent said other end of said first horizontal element, a first contact carried by said first horizontal element, a second switch arm, a second contact carried by said second arm cooperable with said first contact, manually adjustable means engageable with the second switch arm for moving said contacts relative to each other, and thermally responsive means engageable with said struts adjacent their locus of juncture, said struts having a cross-section limited with respect to the current controlled by the contacts for causing substantial expansion and contraction of the struts by heating and cooling in response to the changes in current carried by the struts on engagement and disengagement of the contacts.

5. A thermal switch comprising a first contact arm including a pair of mechanical struts of electrically conductive material connected together at one end, a first strip portion connected at one end to the other end of one strut and a second strip portion connected at one end to the other end of the second strut, the other end of said second strip portion extending to a locus subjacent said one end of the first strip portion and electrical insulation disposed between said one end of the first strip portion and said other end of the second strip portion, a first contact carried by said second strip portion, a second contact arm, a second contact carried by said second arm cooperable with said first contact, thermally responsive means engageable with the connected ends of said struts, said struts having a cross section limited with respect to the current passing through the contacts for causing substantial expansion and contraction of the struts by heating and cooling in response to the changes in current carried by the struts on engagement and disengagement of, the contacts,

6. A thermal switch in accordance with claim 5 in which said thermally responsive means includes a bimetallic member and an upstanding insulating stud carried thereby, the free end of said stud being engageable with the connected ends of said struts.

7. A thermal switch comprising a first contact arm including a horizontal portion supported at one end and a triangular metallic portion supported at the other end, said triangular portion constituting a pair of inclined legs and a horizontal leg with one of said inclined legs being connected to said horizontal portion and the other to said horizontal leg and electrical insulation disposed between the other end of said horizontal leg and said horizontal portions, a first contact carried by the horizontal leg, 21 second contact arm, a second contact carried by said second arm cooperable with said first contact, thermally responsive means engageable with the apex of said triangular portion, said inclined legs having a cross section limited with respect to the current passing through the contacts for causing substantial expansion and contraction of the inclined legs by heating and cooling in response to the changes in current carried by the inclined legs on engagement and disengagement of the contacts.

8. A thermal switch in accordance with claim 7 in which said thermally responsive means includes a bimetallic member and an upstanding insulating stud carried thereby, the free end of said stud being engageable with the apex of said triangular portion.

References Cited in the file of this patent UNITED STATES PATENTS 2,388,033 Berninger Oct. 30, 1945 2,505,689 Rickmeyer Apr. 25, 1950 2,609,466 Blonder Sept. 2, 1952 2,716,173 Russell Aug. 23, 1955 2,716,174 Franklin et al. Aug. 23, 1955 2,716,175 Franklin Aug. 23, 1955 

